1. Field of the Invention
This invention pertains to detection of radiation, and more particularly to a device for detection of radiation which may include mid- and long-wavelength infrared radiation, having wavelength from 2 to 30 xcexcm.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
There are a variety of applications in which it is desirable to convert infrared radiation into electrical signals. Examples of such applications include thermal imaging and transmission of information using signals having infrared wavelengths. Of the various approaches to detection of mid- and long-wavelength infrared radiation having wavelengths from about 2 xcexcm to about 30 xcexcm, one of the most popular approaches is the use of a semiconductor p-n junction. Other approaches, such as the use of bolometers which detect temperature changes caused by incident radiation, or quantum-well intra-subband detectors in which incident radiation causes excitation of electrons between confined energy states of a quantum well, generally provide lower sensitivity and/or slower frequency response.
Operation of a basic semiconductor p-n junction photodetector is illustrated by the energy band diagrams of FIG. 1. An energy band diagram, showing electron energy vs. distance, of a p-n junction under equilibrium conditions is shown in FIG. 1(a). This diagram includes conduction band edge 10, valence band edge 12, and Fermi energy level 14. The energy difference 16 between the conduction and valence band energies is known as the energy gap, or bandgap, of the semiconductor. Because the same semiconductor material is used throughout the junction of FIG. 1(a), this energy gap is shown as constant with distance throughout the junction. Conduction in a semiconductor can generally be described in terms of the movement of electrons in the conduction band (having energy at and above that of conduction band edge 10) and holes in the valence band (having energy at and below that of valence band edge 12). The proximity of Fermi level 14 to conduction band edge 10 on the left side of the junction indicates that this portion of the semiconductor is doped n-type, while the right side of the junction is doped p-type. On the n-type side of the junction the majority carriers are electrons and the minority carriers are holes, while the reverse is true on the p-type side. The p-n junction includes a built-in electric field Eo in the junction region where the conduction and valence band edges are bent. The field exerts a force moving any holes appearing in this junction region to the right (in the direction of the field, as shown by the arrow in FIG. 1(a)), and moving any electrons appearing in the junction region to the left (opposite the direction of the field).
A junction photodetector, or photodiode, is typically operated with the p-n junction reverse-biased, as shown in FIG. 1(b). As in the case of built-in electric field of FIG. 1(a), the larger applied electric field of FIG. 1(b) forces electrons toward the n-type side of the junction and holes toward the p-type side. Electrons and holes may be generated in the junction region by absorption of an incident photon such as photon 18. If photon 18 has energy higher than energy gap 16, absorption of the photon may provide energy to excite an electron from the valence band to the conduction band, creating conduction electron 20 and hole 22. The junction region 24 over which electric field E appears may be considered the absorption region of the photodiode, while the outer neutral n-type and p-type regions may be considered contact regions for collecting the photogenerated carriers and connecting the photodiode to a surrounding circuit. Since electrons are collected on the n-type side and holes on the p-type side, each contact region collects photogenerated majority carriers. It should be noted that the designation of a xe2x80x9cmajorityxe2x80x9d or xe2x80x9cminorityxe2x80x9d carrier is dependent upon the location of the carrier within the device. A hole formed by absorption of a photon on the n-type side of the diode is a minority carrier when formed, and becomes a majority carrier upon being transported by the electric field to the p-type side of the diode.
The absorption and contact regions of the photodetector of FIG. 1 are formed using semiconductor materials. Semiconductor alloys have been used traditionally. In recent years, semiconductor superlattices formed from thin alternating layers of two different semiconductors have been used in photodetector structures. Discussion of superlattice photodiodes may be found, for example, in papers by Johnson et al. (xe2x80x9cElectrical and optical properties of infrared photodiodes using the InAs/Ga1xe2x88x92xInxSb superlattice in heterojunctions with GaSb,xe2x80x9d J. Appl. Phys. 80(2), pp. 1116-1127, 1996), Bxc3xcrkle et al. (xe2x80x9cElectrical characterization of InAs/(GaIn)Sb infrared superlattice photodiodes for the 8 to 12 xcexcm range,xe2x80x9d Mat. Res. Soc. Symp. Proc. Vol. 607, p. 77, 2000), and Fuchs et al. (xe2x80x9cInAs/Ga1xe2x88x92xInxSb infrared superlattice photodiodes for infrared detection,xe2x80x9d Proc. SPIE 3287, p. 14, 1998). The constituent layers of a superlattice as described in these references are thin enough that the energy band structures of the layers interact with each other to form conduction and valence xe2x80x9cminibandsxe2x80x9d extending throughout the superlattice. The lowest-energy conduction miniband may be described as the xe2x80x9ceffective conduction bandxe2x80x9d, the highest-energy valence miniband may be described as the xe2x80x9ceffective valence bandxe2x80x9d, and the energy difference between these bands may be described as the xe2x80x9ceffective energy gapxe2x80x9d, or xe2x80x9ceffective bandgapxe2x80x9d of the superlattice. Energy band diagrams as used herein are intended to represent either actual band structure for alloy material implementations or effective band structure in the case of superlattice implementations. Furthermore, the phrase xe2x80x9cenergy gapxe2x80x9d or xe2x80x9cbandgapxe2x80x9d is intended to also refer to an effective energy gap in the case of superlattice implementations. Similarly, the phrase xe2x80x9ceffective energy gapxe2x80x9d or xe2x80x9ceffective bandgapxe2x80x9d may refer to an actual bandgap in the case of an alloy material implementation.
The efficiency and sensitivity of a photodetector such as that of FIG. 1 can be limited by various processes. For example, carriers can be generated by thermal energy in the semiconductor rather than by photon absorption, creating a xe2x80x9cdark currentxe2x80x9d limiting the ability of the detector to detect low radiation levels. Such thermal generation may be particularly problematic for mid- and long-wavelength infrared detectors, since the energy gaps of such detectors are relatively small, and corresponding thermal generation rates are therefore relatively high. Other carrier generation processes such as Auger generation may also be significant, particularly for narrow-gap semiconductors. Minority carriers can cause problems in the device even when generated by photon absorption, in that such minority carriers may recombine with majority carriers instead of crossing the device to be collected at the appropriate contact. Such recombination causes a loss of signal, reducing the quantum efficiency of the photodiode.
One approach which has been used in attempting to mitigate problems such as described in above is to replace the p-n junction of FIG. 1 with a heterojunction in which one side of the junction is formed from a material having a larger energy gap. Such a p-n heterojunction, as described in U.S. Pat. No. 4,961,098 to Rosbeck et al. and U.S. Pat. No. 5,016,073 to Elliott et al. (hereinafter xe2x80x9cElliottxe2x80x9d), may be advantageous by reducing the thermally-generated dark current of the diode, since thermal generation is greatly reduced in the wide-bandgap portion of the device. It is taught in Elliot that the heterojunction between the p-type and n-type sides of the junction may be graded to prevent conduction barriers caused by heterojunction band discontinuities. An additional approach described in Elliott is forming a heterojunction at the interface between the p-type side of the junction and the p-type contact region, such that the contact region has a larger energy gap than the absorption region. In the photodetector of Elliot this p-p heterojunction is arranged within a minority-carrier (electron, in this case) diffusion length of the above described p-n heterojunction. It is taught in Elliot that the proximity of the two heterojunctions causes a reduction in minority carrier density on the p-type side of the p-n heterojunction, allowing operation at higher temperatures than normally attainable. In a paper by T. Ashley and C. T. Elliot, xe2x80x9cOperation and properties of narrow-gap semiconductor devices near room temperature using non-equilibrium techniques,xe2x80x9d Semicond. Sci. Technol. 6, pp. C99-C105, 1991 (hereinafter xe2x80x9cAshleyxe2x80x9d), a structure similar to that in the Elliot patent is discussed, in which a relatively thin high-bandgap layer is interposed between the p-type side of the junction and the p-type contact layer. The use of such a thin high-bandgap layer allows it to be formed from a strained, non-lattice-matched material.
Although the structures of the above-described references provide at least the advantages noted above, some issues do not appear to be addressed by these structures. For example, the structures described above do not prevent, or provide an impediment to, minority carriers generated in both contact regions from entering the absorption region of the detector. Although the structure of Ashley prevents minority carriers in the p-type contact region from entering the absorption region, this is not the case on the n-type side of the device. Furthermore, the interfaces within structures such as those of Elliott and Ashley may cause increased recombination of photogenerated carriers. It would therefore be desirable to develop a technique or design to simultaneously increase the collection efficiency and reduce the noise current in infrared photodiodes.